Recent advancement of electronic instruments has been accompanied by increasingly strict requirements of wiring boards for packaging semiconductor (hereinafter, will be referred to as semiconductor-packaging wiring boards), such as PGA (pin grid array) or BGA (ball grid array) packages, for high wiring density, moisture resistance and heat resistance. Also, the heat generated per unit area has been increased due to increasing electronic component mounting density and increasing integration of semiconductor devices, requiring efficient heat dissipation from the semiconductor-packaging wiring boards. Common means for accelerating the dissipation of heat generated by electronic components mounted on semiconductor-packaging wiring boards are the use of wiring boards which are composed of a metal plate bonded with copper foil by insulating adhesives, or printed wiring boards bonded to radiation plates of high heat conductivity, such as aluminum plates, copper plates or steel plates.
To meet these requirements, these wiring boards require complicated shape-processing, such as multilayering, formation of cavities or installation of radiation plates.
Bonding materials commonly used to produce semiconductor-packaging multilayer wiring boards or to bond radiation plates are glass cloth-epoxy resin prepregs containing glass fibers base materials, and rubber-epoxy resin adhesives. When radiation plates are bonded with semiconductor-packaging wiring boards by using glass cloth-epoxy resin prepregs, the wiring boards tend to warp or form CAF (Conductive Anodic Filaments), which occur due to migration of copper ion along the interface between glass fibers and resin, and, on moisture absorption, become poor in soldering heat resistance and electrolytic corrosion resistance. Further, the use of glass cloth-epoxy resin prepregs in the production of wiring boards requiring complicated shape-processing, such as formation of cavities or installation of radiation plates, causes the disadvantage that epoxy resin powder is scattered during drilling to coat connecting surfaces, causing connecting error in wire bonding, or the resin flows in large quantities and even into cavities by the heat and pressure applied during lamination, so that the cavities lose necessary spaces.
Rubber-epoxy resin adhesives are mixtures of epoxy resins with various rubbers components, such as acrylic rubbers or acrylonitrile-butadiene rubbers, which are added to improve the adhesives in strength, flexibility and adhesion. Because of their relative excellency in properties, such as electrical properties, heat resistance and solvent resistance, these conventional acrylic rubber-containing adhesives have been used in the production of flexible boards.
Typical examples of acrylic rubber-containing adhesives are adhesive sheets made of mixtures of acrylic rubber with thermosetting resins, such as epoxy resins, which, however, exhibit insufficient properties at high temperatures due to the low epoxy resin content of about 30% by weight. Further, these adhesives composed mainly of acrylic rubbers have the defects that though the loss of adhesion strength after a long term treatment at high temperatures is relatively small, the adhesion strength at high temperatures is poor, and that the properties, such as heat resistance and electric properties, are considerably deteriorated on moisture absorption. Adhesives composed mainly of acrylonitrile-butadiene rubbers have the defect that the loss of adhesion strength after a long term treatment at high temperatures is considerable and they are inferior in electrical corrosion resistance. The deterioration is particularly significant when they are subjected to tests for moisture resistance under today's severe conditions to which electronic instruments are exposed, such as PCT (pressure cooker test) treatments.
As adhesives improved in the soldering resistance after moisture absorption, in Japanese Patent Application Unexamined Publication No. 60-243180 (1985) are disclosed adhesives containing acrylic resins, epoxy resins, polyisocyanates and inorganic fillers, in Japanese Patent Application Unexamined Publication No. 61-138680 are disclosed adhesives containing acrylic resins, epoxy resins, compounds having urethane bonds in molecule and primary amines at both ends. These adhesives, however, are unsatisfactory since they are considerably deteriorated when subjected to tests for moisture resistance under today's severe conditions to which electronic instruments are exposed, such as PCT.
Rubber-epoxy resin adhesives improved in adhesion at high temperatures are mixtures of reactive rubber components, such as reactive acrylic rubbers or reactive acrylonitrile-butadiene rubbers, with epoxy resins. Examples of such reactive rubber-epoxy resin adhesives are adhesive compositions disclosed in Japanese Patent Application Unexamined Publication No. 3-181580 (1991), which comprise acrylic rubbers having carboxylic groups, hydroxyl groups or epoxy groups, alkyl phenols, epoxy resins and imidazolium trimellitate as an cure accelerator, and are used to bond base film of flexible printed wiring boards with copper foil. In these adhesive compositions, the carboxyl group-containing acrylic rubbers, the hydroxyl group-containing acrylic rubbers and the epoxy group-containing acrylic rubbers are used individually, and compositions containing two or more of them are not disclosed.
As adhesive compositions improved in adhesion to shiny side of copper foil and in heat resistance, in Japanese Patent Application Unexamined Publication No. 7-76679 (1995) are disclosed adhesive compositions which are used in sheet heaters having resistance circuits and comprise 60 to 80 parts by weight of epoxy group-containing acrylic rubbers, 8 to 20 parts by weight of alkyl phenols, 8 to 20 parts by weight of epoxy resins and 0.2 to 1.0 parts by weight of imidazole curing agents, and in Japanese Patent Application Unexamined Publication No. 7-173449 (1995) are disclosed epoxy group-containing acrylic rubber adhesive compositions composed mainly of epoxy group-containing acrylic rubbers.
The above adhesives disclosed in Japanese Patent Application Unexamined Publication Nos. 3-181580, 7-76679 and 7-173449 are not applicable for semiconductor-packaging wiring boards, such as PGA or BGA packages, because they are insufficient in moisture resistance, heat resistance and adhesion strength at high temperatures and, in a moistened state, considerably deteriorated in properties, particularly when subjected to moisture resistance tests under severe conditions, such as PCT treatments.
Additionally, these adhesives need a large ratio of high-molecular weight components contributing flexibility because as the ratio of low-molecular weight components, such as epoxy resins, increases, the degree of cross-linking increases and lowers the flexibility of film made therefrom. The increase of the ratio of the high-molecular weight components, however, inevitably decreases the ratio of the low-molecular weight components having good flowability, to decrease the flowability of the adhesives. These adhesives, even with a low flowability, are applicable without practical troubles in flexible wiring boards, but when used for producing semiconductor-packaging wiring boards, such as PGA or BGA packages, they cannot contribute properties enough for practical uses. That is, the requirements of semiconductor package-mounting wiring boards for flatness of board surfaces, circuit filling and adhesion of adhesives with board surfaces bearing blackened or etched copper foil are too strict for these conventional adhesives of poor flowability to satisfy any one of the requirements for flatness, circuit filling and adhesion.
Further, with the recent increases of the integration and wiring density in IC in PGA and BGA packages, which are more significant than in common wiring boards, heat management has become very important, but there have been no adhesive films designed for heat dissipation.
Among conventional adhesives which are mixtures of acrylic rubber and epoxy resin, those with the greater ratio of acrylic rubbers are defective in moisture resistance, and are revealed to be deteriorated in properties particularly by a PCT moistening treatment at 121.degree. C. due to their low cross-linking density. Adhesive sheets formed from those with the greater ratio of epoxy resins are fragile and tacky due to low strength, low flexibility and high tackiness, and are difficult to handle. Another problem is such a poor cracking resistance that they crack on curing.
In the methods of producing semiconductor-packaging wiring boards, such as PGA or BGA packages, wherein lamination is carried out after adhesive sheets cured to B-stage are drilled to form cavities or IVH (Interstitial Via Hall), it is necessary to reduce the outflow of resins from the drilled edges at the time of lamination with heat and pressure. The resins, however, need some flowability to bury the circuits formed in the wiring boards without leaving voids. Conventional adhesives become more flowable on lamination as the epoxy resin content increases, and cause the troubles of exudation into cavities or IVH, interlaminar connection failure due to the adhesives which cover the circuits formed in the holes, and a decrease of interlaminar insulation distance due to the decreased thickness of insulation layers formed from the adhesives. As the acrylic rubber content increases, circuit filling properties decrease. Therefore, it has been difficult to satisfy both the requirements for the control of exudation due to flowability (control of flowability) and the circuit filling properties (improvement of flowability).
As described above, there have been provided no adhesives satisfying the moisture resistance, heat resistance, adhesive strength at high temperatures, proper flowability and circuit filling that are required of adhesives used in semiconductor-packaging wiring boards, such as PGA or BGA packages.